A load drive system for driving a load such as an electric motor includes a direct-current (DC) power supply, a step-up or step-down DC/DC converter, an inverter which transforms direct-current (DC) power into alternating-current (AC) power, a DC link capacitor which is provided between the DC/DC converter and the inverter for smoothing a DC voltage and a load. In the load drive system, the DC/DC converter, the DC link capacitor and the inverter may be configured as a module. In this case, a current from the DC/DC converter to the DC link capacitor and a current from the DC link capacitor to the inverter influence each other, whereby a ripple current which flows in the DC link capacitor is increased. In view of the lifetime of the DC link capacitor, a loss generated in the DC link capacitor and the size of the module, it is desirable that the ripple current is small.
FIG. 7 illustrates the configurations of a motor drive system and an inverter in the motor drive system. FIG. 8 illustrates operating waveforms when carrier signals of the inverter and the DC/DC converter are optimized. A control circuit 60 for the motor drive system illustrated in FIG. 7 controls so as to synchronize a frequency of an inverter carrier signal for driving a PWM (Pulse Width Modulation) inverter 20 of a triangular wave comparison system with a frequency of a DC/DC converter carrier signal for driving a DC/DC converter 40 so that a center of a time period during which an input current Ip to the inverter 20 becomes zero and a center of a time period during which an output current Io from the DC/DC converter 40 coincide with each other. In an example illustrated in FIG. 8, the frequency of the DC/DC converter carrier signal is controlled to be twice the frequency of the inverter carrier signal for synchronization. A pulse timing of the current flowing from the DC/DC converter to the DC link capacitor coincides with a pulse timing of the current flowing from the DC link capacitor to the inverter. Therefore, a ripple current Icap which flows in the DC link capacitor 30 becomes small.